The inventions described herein may be manufactured, used and licensed by or for the U.S. Government for U.S. Government purposes.
1. Field of the Invention
The present invention relates generally to sensors, and more particularly, to high-g hardened accelerometer sensors.
2. Prior Art
The state of art in shock resistant accelerometer design is to reduce the size of the moving proof mass, thereby reducing the related forces, moments, and torques that are generated in the presence of high acceleration levels, i.e., when the accelerometer experiences shock or impact loading, thereby making it possible to provide stops in the path of the moving component(s) of the accelerometer to limit its maximum deflection. The introduction of MEMS technology in recent years has made it possible to reduce the size of the proof mass significantly, independent of the accelerometer type and its mechanism of operation. All existing accelerometer designs, however, suffer from the following operational and/or performance deficiencies.
There is a significant amount of settling time required for the accelerometer to settle within an acceptable level following shock loading. Many types of sensors, particularly accelerometers, rely upon the deflection of one or more structural elements of the sensor, which due to the high-g loading and the inevitable mass (inertia) of the structural element, a deflection results. In a high-g state, the deflection results in a vibration of the structural element. The time until the vibration ceases or reduces to an acceptable value is referred to as a settling time. The settling time is particularly important for accelerometers that are used as IMUs on guns or similarly fired projectiles and that are intended to be used for navigation and/or guidance and/or control.
For accelerometers that are designed without proof mass limit stops, accelerometer designs that allow high low-g sensitivity while can tolerate high-g shock loads without permanent damage or change in their characteristics, are yet to be conceived. Due to the nature of all proof mass based accelerometers, high sensitivity to low acceleration levels make them highly susceptible to shock loading damage since they rely on relatively large deformations to be induced in the accelerometer mechanism due to small input accelerations.
Therefore, there is a need in the art for sensors, in particularly accelerometers, which are sensitive enough to provide accurate sensing of a desired parameter, such as acceleration, yet rugged enough to withstand shock loading due to an external stimulus such as a high acceleration. Furthermore, there is a need in the art for sensors, in particularly accelerometers, in which the settling time of a deflected member is minimized.
Therefore it is an object of the present invention to provide such a sensor that overcomes the aforementioned problems of the prior art.
Accordingly, a sensor is provided. The sensor comprises: a base; at least one component which moves relative to the base; and locking means for locking the at least one component in a predetermined stationary position in response to an external stimulus. Preferably, the sensor is an accelerometer in which case the external stimulus is preferably an acceleration of the sensor and the locking means locks the at least one component during periods in which the acceleration exceeds a predetermined value.
In a first variation of the sensor, the predetermined stationary position comprises a null position taken by the at least one component corresponding to an acceleration that is substantially zero. In an alternative variation of the sensor, the predetermined stationary position comprises an active position taken by the at least one component when acceleration equals a predicted acceleration other than zero.
In a first variation of the locking means of the sensor, the locking means preferably comprises an active means for locking the at least one component in the predetermined stationary position in response to the external stimulus. In which case, the locking means preferably comprises: at least one movable member movably disposed on the base and configured to engage at least a portion of the at least one component; means for generating a lock signal in response to the external stimulus and an unlock signal in response to the absence of the external stimulus; and an actuator for moving the at least one movable member into engagement with the at least one component to lock the at least one component in response to the lock signal and to unlock the at least one component in response to the unlock signal.
In a second variation of the locking means of the sensor, the locking means comprises a passive means for locking the at least one component in the predetermined stationary position in response to the external stimulus. In which case, the locking means comprises: a first locking mass movably disposed about the base in a first direction; and a second locking mass movably disposed about the base in a second direction opposite the first direction; wherein the first and second locking masses move to sandwich the at least one component therebetween in response to the external stimulus. In the second variation, the locking means further preferably comprises a locking stop positioned between the first and second locking masses and corresponding to the predetermined stationary position, wherein the locking stop and the at least one component are sandwiched between the first and second locking masses in response to the external stimulus.
Also provided is a method for protecting a sensor from an external stimulus. The method comprises: providing a sensor having a base and at least one component which moves relative to the base; and locking the at least one component in a predetermined stationary position in response to the external stimulus.
Where the external stimulus is an acceleration of the sensor, in a first variation the locking comprises locking the at least one component in a null position taken by the at least one component corresponding to an acceleration that is substantially zero.
Where the external stimulus is an acceleration of the sensor; in a second variation the locking comprises locking the at least one component in an active position taken by the at least one component when acceleration equals a predicted acceleration other than zero.
Where the external stimulus is an acceleration of the sensor, the locking preferably comprises locking the at least one component during periods in which the acceleration exceeds a predetermined value.
In a first variation of the locking, the locking comprises actively locking the at least one component in the predetermined stationary position in response to the external stimulus. In which case, the locking comprises: providing at least one movable member movably disposed on the base and configured to engage at least a portion of the at least one component; generating a lock signal in response to the external stimulus and an unlock signal in response to the absence of the external stimulus; and moving the at least one movable member into engagement with the at least one component to lock the at least one component in response to the lock signal and to unlock the at least one component in response to the unlock signal.
In a second variation of the locking, the locking comprises passively locking the at least one component in the predetermined stationary position in response to the external stimulus. In which case, the locking comprises: providing a first locking mass movably disposed about the base in a first direction and a second locking mass movably disposed about the base in a second direction opposite the first direction; and passively moving the first and second locking masses to sandwich the at least one component therebetween in response to the external stimulus. Preferably, the locking of the second variation further comprises: providing a locking stop positioned between the first and second locking masses and corresponding to the predetermined stationary position; and sandwiching the locking stop and the at least one component between the first and second locking masses in response to the external stimulus.